Such an actuator is known from U.S. Pat. No. 6,476,702, for example. This actuator has an electrically conductive coil, which has a longitudinal axis and a plurality of turns, and including at least one magnet, which is arranged spaced apart from the turns of the coil in the radial direction R with respect to the longitudinal axis L. The coil is at least partially covered on a side remote from the magnet by a central region of a first conducting element, and the at least one magnet is at least partially covered on a side remote from the turns of the coil by a central region of a second conducting element. The first conducting element protrudes beyond the coil, and the second conducting element protrudes beyond the at least one magnet, in the axial direction with respect to the longitudinal axis L, and the first and second conducting elements each have collar-like projections there. Such actuators contain an oscillatory mass-spring system, which is excited so as to produce oscillations when an alternating current is driven through the turns of the electrically conductive coil.
The at least one magnet has a magnetization with a magnetization direction which is ideally perpendicular to the longitudinal axis of the coil. If a current now flows through the coil, a Lorentz force acts in the direction of the longitudinal axis of the coil. As described in U.S. Pat. No. 6,476,702, the interaction of the magnetic lines of force emerging from the collar-like projections of the first conducting element and the second conducting element or the magnetization of the magnet, which advantageously likewise consists of a material of high permeability, results in a further force, which acts in the same direction as the described Lorentz force. Since the magnetic lines of force are conducted in specific directions by the first and second conducting elements, both component parts are in this case referred to as conducting elements.
In the case of an actuator of the generic type, either the coil with the first conducting element or the magnet with the second conducting element is mounted in a sprung manner, while the corresponding other assembly is mounted statically. If a current now flows through the coil, the abovementioned forces result in a shift in the spring-mounted assembly and therefore in a movement of the actuator. In this way, valves can be opened or closed, for example.
If an alternating current flows through the coil instead of the direct current, the direction of the acting forces reverses along with the current flow direction. In this way, the spring-mounted assembly is caused to oscillate. By targeted selection of the amplitude, frequency and phase of the applied alternating current, the oscillation of the actuator can be controlled very precisely. In this way, for example, oscillations can be produced or an oscillation in phase opposition can be superimposed on already existing oscillations and these already existing oscillations can thus be compensated for.
One disadvantage is the fact that the excitation force that can be achieved with such an actuator in accordance with the prior art is relatively low in relation to the physical volume required for this.
It is also disadvantageous that the usable frequency range in which such an actuator can be operated on alternating current is subject to restrictions. Firstly, the first natural frequency of the system, which is the lowest frequency at which the actuator can be operated, cannot be shifted to lower frequencies. Secondly, the working range in the higher-frequency range is limited owing to the low amount of coil installation space and the use of coils with a small wire diameter associated therewith.
A further disadvantage when using alternating current consists in that, as the frequency of the current increases, the inductive reactance of the coil and therefore also the total impedance increase. On a predetermined voltage of 12 V, for example, as prevails in the electric power supply system of a typical passenger vehicle, the level of the drivable current is therefore reduced depending on, inter alia, the inductance and the resistance of the coil and the operating frequency of the actuator. In this case, the substantial disadvantage primarily consists in the high inductance of the coil used which has an increasingly negative effect on the performance capacity of the actuator, that is, the achievable dynamic force at a certain operating frequency, as the frequency increases and as a result of the surrounding “iron jacket”.